Documentation

A Functoid is either a FunctoidParts, representing a partially
defined function, or a (completely defined) Function.
A FunctoidParts represents an incompletely defined function,
which has a function name, argument names, and a list of graph nodes
representing the function body which might not yet form
a complete expression tree, e.g., nodes might
be unconnected.

What is a FunctoidLayout? You can think of it as a
tree layout for a Frame,
or for a Functoid, or as a Forest of Tree Layouts!
For an Edit frame, it must be a forest, since the nodes are not
yet necessarily connected into a tree.
For a call frame, it will be a single TreeLayout (singleton list).